Recent European marine heatwaves are unprecedented but not unexpected

The European North-West shelf seas experienced a marine heatwave of unprecedented magnitude in June 2023. Quantifying the likelihood of reoccurrence of similar events is vital for mitigating impacts on marine ecosystems and human activities. Assessing the probability of such events is complicated by climate change-driven changes in the baseline conditions and the short length of the observational record with respect to modes of climate variability. Here, by employing a large ensemble of initialised climate model simulations, we show that the probability of June 2023-like events occurring is approximately 10% in any given year of the present-day climate. Moreover, there has been accelerating growth in the risk of occurrence over the last 30 years. The unprecedented nature of the record-breaking June 2023 event placed European marine heatwaves firmly in the public consciousness. However, the climate change trajectory means that whilst this event was unprecedented, such events should not be unexpected.